1. Field of the Invention
The present invention relates to an electrophotographic photoconductor (hereafter also referred to simply as “photoconductor”) which has a photoconductive layer containing an organic material and which is used in electrophotographic printers, copiers, fax machines and the like, to a method for producing the electrophotographic photoconductor, and to an electrophotographic device. In particular, the invention relates to a multilayer-type or single-layer-type electrophotographic photoconductor having excellent image characteristics and electric characteristics, through improvement of a resin binder that is a constituent material of a photoconductive layer, to a method for producing the electrophotographic photoconductor, and to an electrophotographic device.
2. Description of the Related Art
The various functions that are required of electrophotographic photoconductors include, ordinarily, a function of holding surface charge, in the dark, a function of generating charge through reception of light, and a function of transporting charge likewise through reception of light. Such electrophotographic photoconductors include so-called multilayer-type photoconductors having a stack of layers functionally separated into a layer that contributes mainly to charge generation, and a layer that contributes to holding surface charge, in the dark, and to charge transport upon light reception, and so-called single-layer-type photoconductors wherein these functions are combined in one layer.
For instance, the Carlson method is used in image formation by an electrophotographic method that utilizes such electrophotographic photoconductors. Image formation according to this scheme involves charging a photoconductor in the dark, forming a electrostatic latent image corresponding to text, pictures or the like of an original, on the charged photoconductor surface, through exposure, developing the formed electrostatic latent image by means of toner, and transferring and fixing the developed toner image onto a support such as paper. The photoconductor after toner image transfer undergoes, for instance, removal of residual toner and removal of charge, and is thereafter re-used.
Materials used as the above-described electrophotographic photoconductor include inorganic photoconductive materials such as selenium, selenium alloys, zinc oxide and cadmium sulfide. In electrophotographic photoconductors that have been brought to practical use in recent years, a photoconductive layer is formed by dispersing, in a resin binder, an organic photoconductive material, which is advantageous in terms of thermal stability, film-forming properties and so forth, in comparison to inorganic photoconductive materials. Examples of such organic photoconductive materials include, for instance, poly-N-vinyl carbazole, 9,10-anthracenediol polyester, pyrazoline, hydrazone, stilbene, butadiene, benzidine, phthalocyanines and bisazo compounds.
In recent years, the above-described functional separation multilayer-type photoconductors, having formed therein a photoconductive layer that is a stack of a charge generation layer containing a charge generation material and a charge transport layer containing a charge transport material, have entered the mainstream thanks to the large degree of design freedom that these photoconductors afford, by virtue of the broad selection of materials, from among a wealth of organic materials, that are suited for the various functions of the photoconductive layer.
From among the foregoing photoconductors, numerous products have been made out of negative charging-type photoconductors having a charge generation layer in the form of a conductive substrate on which there is formed a layer of an organic photoconductive material, by vapor deposition, or a layer that is formed by dip coating in a coating solution having an organic photoconductive material dispersed in a resin binder, the photoconductor having, on the charge generation layer, a charge transport layer in the form of a layer that is formed by dip coating using a coating solution resulting from dispersing or dissolving, in a resin binder, an organic low-molecular compound having a charge transport function.
Positive charging-type photoconductors that rely on a single photoconductive layer formed by dispersing or dissolving a charge generation material and a charge transport material in a resin binder are likewise well known.
Electrophotographic printing devices are required to possess, among others, ever higher durability and sensitivity, and faster response, to cope with, for instance, increases in the number of copies to be printed in a networked office, and to cope with the rapid development of lightweight electrophotographic printing machines. These devices, moreover, are held to strict requirements in terms of exhibiting little fluctuation in image characteristics and electric characteristics that arise from repeated use and from variations in the usage environment (room temperature, humidity).
The development and growing spread of color printers in recent times have been accompanied by greater printing speeds, smaller devices, fewer components, and the need to cope with a variety of usage environments. In color printers, a tendency is observed wherein transfer current increases on account of toner color overlap and/or the use of transfer belts. When printing on paper of various sizes, a difference in transfer fatigue arises between portions with paper and portions without paper. This in turn exacerbates differences in image density, which is problematic. In case of frequent printing on small-sized paper, bare photoconductor portions over which the paper does not pass (paper non-passage sections) are continuously and directly affected by transfer, and exhibit greater transfer fatigue than photoconductor portions over which paper does pass (paper passage sections). As a result, when printing is performed next on large-size paper, the abovementioned discrepancy in transfer fatigue between paper passage sections and paper non-passage sections gives rise to a potential difference in the developed area, which translates into differences in density. This trend becomes yet more pronounced as transfer current increases. Under such circumstances, the demand has intensified for photoconductors that exhibit little fluctuation in image characteristics and electric characteristics as a result of repeated use, or on account of fluctuations in the usage environment (room temperature and environment), and that exhibit excellent transfer resiliency, particularly in color printer, as compared to monochrome printers. However, conventional technologies have thus far failed to meet these requirements simultaneously to a sufficient degree.
As described above, the charge generation layer is ordinarily formed as a layer that comprises a dispersion of a charge generation material in the form of an organic photoconductive material, such as a phthalocyanine compound, in a resin binder. Various types of resins have been considered as such a resin binder.
For instance, polyvinyl acetal resins and polyvinyl butyral resins exhibit good pigment dispersibility in the coating solution during the production of the photoconductor, and are excellent in adhesiveness, as disclosed in Patent Document 1 (Japanese Patent Application Publication No. S62-95537) and Patent Document 2 (Japanese Patent Application Publication No. S58-105154). The synthesis method of polyvinyl acetal resins themselves has also been the object of study, as disclosed in Patent Document 3 (Japanese Patent Application Publication No. H5-1108).
Patent Document 4 (Japanese Patent Application Publication No. 2006-133701) studies a charge generation layer that contains, in specific mixing ratios, two polyvinyl butyral resins having dissimilar degrees of butyralization, and two polyvinyl butyral resins having dissimilar contents of hydroxyl groups. The charge generation layer is found to be effective in improving repeat stability and sensitivity under high-temperature, high-humidity environments, but the transfer resistance of the charge generation layer is not addressed.
Also known are technologies for enhancing sensitivity, repeat durability and liquid storage stability by, for instance, combining a polyamide, as a binder for an undercoat layer binder, with a polyvinyl butyral resin, as a binder for a charge generation layer (Patent Document 5, Japanese Patent Application Publication No. S58-30757), or combining a copolymer nylon, as a binder for an undercoat layer, with a polyvinyl butyral resin, as a binder for a charge generation layer (Patent Document 6, Japanese Patent Application Publication No. H9-265202). Transfer resistance, however, is not addressed. Patent Document 7 (Japanese Patent Application Publication No. 2001-105546) discloses a laminate that comprises a base material layer and a layer of a curable resin composition that comprises a specific modified polyvinyl acetal resin, and discloses a specific example relating to a polyvinyl acetal resin comprising phenyl groups (butyl groups:phenyl groups=19:59), but the laminate does not pertain to a photoconductor.
As described above, polyvinyl acetal resins including polyvinyl butyral resins are known constituent materials in photoconductive layers of electrophotographic photoconductors, and methods for producing and using these resins have been the object of various studies. However, none of those studies has succeeded thus far in sufficiently satisfying all from among high transfer resistance, high memory characteristics and good electric characteristics.
Accordingly, it is an object of the present invention to solve the above issues and to provide an electrophotographic photoconductor having high transfer resistance, high memory characteristics and good electric characteristics, to provide a production method of the electrophotographic photoconductor, and to provide an electrophotographic device.